Temperature sensing circuits are used in various electronic devices, such as voltage regulators, personal computers, and various kinds of portable devices and home appliances, where control is performed in response to changes in ambient temperature.
FIG. 1 is a diagram illustrating an example of a conventional temperature sensing circuit.
As shown in FIG. 1, the conventional circuit includes a comparator 30, a reference voltage Vr, diodes D1 and D2, and a constant current source I1.
In the temperature sensing circuit, the constant current source I1 and the diodes D1 and D2 are connected in series between a voltage source Vdd and ground, forming a node N1 between the current source I1 and the diode D1. The comparator 30 has a non-inverting input connected to the node N1, an inverting input connected to the reference voltage Vr, and an output to provide a temperature detection signal Out.
During operation, the comparator 30 compares a voltage drop across the diodes D1 and D2 against the reference voltage Vr. The voltage Vr is generated by an appropriate source (e.g., a bandgap regulator) having a good temperature coefficient. The comparator output Out switches according to whether the voltage drop is above or below the reference voltage Vr.
The above temperature sensing circuit is designed to take advantage of the fact that the voltage drop across the series diodes D1 and D2 biased with the constant current I1 has a temperature coefficient. However, such a conventional design involves various electronic components for implementing various functions, such as pn junction diodes for the series diodes D1 and D2, a voltage regulator for the reference voltage source Vr, and other elements for the comparator 30, leading to increased size and complexity of the temperature sensing circuit.
By contrast, instead of using a voltage drop across coupled diodes, some recent techniques provide temperature sensing capabilities through use of a difference in gate work function between metal-oxide-semiconductor field-effect transistors (MOSFETs) with a controlled temperature coefficient.
FIG. 2 is a block diagram schematically illustrating an example of such a temperature sensing circuit.
As shown in FIG. 2, the temperature sensing circuit includes a first voltage generator 101, a second voltage generator 102, a subtractor 103, and a comparator 104.
The first voltage generator 101 generates a voltage Svptat proportional to absolute temperature (PTAT) and hence having a linear temperature coefficient either positive or negative. The second voltage generator 102 generates a first reference voltage Vref, a second reference voltage Tvref, and a third reference voltage Svref, all having no temperature coefficient.
The subtractor 103 amplifies a difference between the voltage Svptat and the third reference voltage Svref to provide an output Tvptat to the comparator 104. The comparator 104 then compares the signal Tvptat against the second reference voltage Tvref to output a temperature detection signal Tout.
In such a configuration, the second voltage generator 102 providing a voltage with no temperature coefficient operates based on a difference in gate work function between multiple FETs.
FIG. 3 is a diagram illustrating still another example of temperature sensing circuit.
As shown in FIG. 3, the temperature sensing circuit includes a first voltage generator 201, a second voltage generator 202, an impedance transformer 203, and a subtractor 204.
The first voltage generator 201 generates an output voltage VPN with a negative temperature coefficient based on a difference in gate work function between a pair of FETs.
The second voltage generator 202 generates a reference voltage VREF1 with no temperature coefficient based on a difference in gate work function between multiple FETs.
The impedance transformer 203 includes first and second operational amplifiers (op-amps) AMP1 and AMP2, and performs impedance transformation on the signals VPN and VREF1 prior to transmission to the subtractor 204.
In the impedance transformer 203, the first and second op-amps AMP1 and AMP2 each forms a voltage follower with an output connected to an inverting input. The first op-amp AMP1 receives the voltage VPN at a non-inverting input and provides a low-impedance output to one input terminal of the subtractor 204. Similarly, the second op-amp AMP2 receives the voltage VREF1 at a non-inverting input and provides a low-impedance output to another input terminal of the subtractor 204.
The subtractor 204 includes an op-amp AMP and resistors R1 through R4, and provides a temperature detection signal VOUT at an output of the op-amp AMP.
In the subtractor 204, the op-amp AMP receives the reference voltage VREF1 at a non-inverting input via the resistor R1 and the voltage VPN at an inverting input via the resistor R3, with the resistor R2 connected between the non-inverting input and ground, and the resistor R4 connected between the output and inverting input. The temperature detection signal VOUT is generated through subtraction between the input voltages VREF1 and VPN.
In such a configuration, a voltage VREF1-VPN obtained by subtracting the negative-temperature-coefficient voltage VPN from the no-temperature-coefficient voltage VREF1 has a positive temperature coefficient. Thus, the detection signal VOUT obtained by amplifying VRFF1-VPN also has a positive temperature coefficient greater than that of the difference voltage VRFF1-VPN, which provides good detection accuracy and low energy consumption of the temperature sensing circuit.
Although providing temperature sensing capabilities without using diodes, the MOSFET-based approaches illustrated in FIGS. 2 and 3 do not provide a satisfactory reduction in circuit size, since these circuits require two voltage generators, one with a temperature coefficient and the other with no temperature coefficient, in addition to a comparator for comparing the outputs of the voltage generators.
Accordingly, there remains a need for a temperature sensing circuit that provides a good temperature detection performance in a simple and compact circuit configuration. Such a circuit will contribute to a size reduction of various electronic devices incorporating temperature sensing capabilities.